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July-August 2009

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The Mineralogy of the Buck Hill Syenite Intrusion

Augusta County, Virginia

Buck Hill is a small, elevated ridge located just east of the George Washington National Forest on the 7.5-minute Stokesville Quadrangle, Augusta County, Virginia. In this area, forty-one dikelike intrusions have been mapped (Rader 1969). These intrusives consist of a suite of silica-deficient, alkalic rocks (teschenite, ijolite, and syenite) of Jurassic-Cretaceous age. The local geology is composed of Upper Ordovician, Silurian, and Lower Devonian carbonates and clastics that are regionally folded (Alleghenian Orogeny)and faulted by the Little North Mountain thrust fault (Rader 1969; Johnson, Milton, and Dennison 1971; Fitzgerald 1966). The area intrusives are spatially related to a larger suite of diverse, mafic to alkaline igneous rocks found in Augusta, Highland, and Rockingham counties, Virginia, and bordering Pendleton County, West Virginia (Johnson 1965). These igneous rocks are Upper Cretaceous through Tertiary in age (Fullagar and Bottino 1969; Tso et al. 2003) and represent the youngest igneous activity known in the eastern United States. The exact origin and relationship of these rocks to the regional tectonics is not fully understood.he occurrence of small cubic crystals of “fluate of lime” (fluorite) in rocks “near the falls of Niagara.” According to Bruce, this was the first documented occurrence of fluorite in the United States: “The fluate of lime, though well known in many parts of Europe, has hitherto been unnoticed in the United States” (Bruce 1808, p. 442).
Dr. Lance E. Kearns is a mineralogist, professor of geology, and curator of the Mineral Museum at James Madison University in Harrisonburg, Virginia. Frank Coccina has a master’s degree in geology and works for a large consulting firm in Los Alamos, New Mexico. Alyssa Saint John has a master’s degree in geology and lives and works in Richmond, Virginia. Erin Raiter taught physical geology and Earth science in Freedom High School, Woodbridge, Virginia, last year. She is currently in graduate school in Dearborn, Michigan. Thomas Tucker, a geologist and mineral enthusiast, is president of the Micromineralogists of the National Capitol Area.

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